Actually sas stores dates as numbers

eg:: given date  12-07-2009


in sas :: we can store like this `12-07-2009`d

we can store date values using date format 
(ex:date9.,ddmmyyyy10.)

We can read the date value with the help of date informats.
Say we have the date 09/13/2008.
Now if we want SAS to read this date, we will use the date 
informat in the input statement like
"input date mmddyy10."
SAS will store this date in the numerical form as the 
number of days passed from 1st january, 1960.

If you don't know in advance how the date will be 
formatted, you can use the ANYDTDTE. informat. This will 
extract a SAS date from most date/date-time formats. 
(Warning: ANYDTDTE doesn't fully support day-month-year 
ordering, even if you have this set as your system 
preference, but it still copes with most DMY-formatted 
dates.)

e.g the date is '05/09/2007'

we can use the input function to convert this string into SAS date i.e.
date='05/09/2007'
sasdate=input(date,mmddyy10.);

Also, we can use mdy function.For this first use substr function.

m=substr(date,4,2);
d=substr(date,1,2);
y=substr(date,7,4);
sasdate=mdy(m,d,y);

For example if we have a date="14/12/2008".

you have to input it first and the format it like
date1=input(date,ddmmyy10.);
format date1 ddmmyy10.;
run;

data temp;
date='17/12/2009';
m=substr(date,4,2);
d=substr(date,1,2);
y=substr(date,7,4);
d1=mdy(m,d,y);
format d1 mmddyy10.;
run;